To refill an ink-jet cartridge from an external tank, a connection pipe is conventionally used which conveys the refilling liquid to an inlet port on a top of the cartridge or cartridge cover. If a porous material is used as a backpressure element to generate backpressure, the cartridge and the porous material are not completely filled up with ink, but an upper internal volume of the cartridge and backpressure element is in contact with a gas, usually air, and/or the outside.
The ink injected by the connection pipe drops down onto the porous material of the backpressure element and flows through the backpressure element finally arriving at a print head chip, where the droplet ejection of the print head takes place.
Even if the ink injected through the connection pipe has been outgassed, the upper internal volume of the cartridge and an upper portion of the backpressure element are filled with air at atmospheric pressure and, hence, the ink passing through the upper volume and upper portion re-captures gas present in these internal parts of the cartridge.
Among the issues that can compromise the correct working of an ink-jet print head, the growing of gas bubbles in the cartridge is a very harmful and shifty one: large size bubbles can heavily hamper the ink flow toward the ejection sites of the print head and even break it off completely.
In each ejection site of a thermal print head, where droplets are formed at the ejection rate, a current pulse through a heating resistor generates a thin vapor layer with an internal pressure of about 9 MPa. This high pressure, imparted to the neighboring liquid, is maintained for a very short time of usually less than 1 μs. Subsequently, the thermal bubble expansion causes the pressure to drop down rapidly and well below the atmospheric level. Such a strong depression (about −80 kPa) holds out for nearly the entire bubble evolution. In other words, the bubble is “seen” by the neighboring ink as a volume being in strong depression most of the time.
In static conditions, the ink is in equilibrium with its environment and it is nearly saturated with the dissolved gas. When the sudden depression occurs, this equilibrium is broken and part of the dissolved gas is extracted from the neighboring ink. After the collapse of the thermal bubble, such extracted gas remains inside the liquid in form of micro-bubbles of air. Due to continuous boiling action, these bubbles are pushed away from the chamber and some of them flow upstream in the standpipe, conveying the ink from the housing of the cartridge to the print head, where the ink is hardly affected by the pressure variations and, thus, remains substantially in equilibrium with the dissolved gas. Therefore, the air bubbles pushed upstream in the standpipe cannot be re-absorbed by a nearly saturated liquid and dwell in the flow path, e.g. below a filter between the housing and the standpipe.
Progressive printing activity can extract more gas from the ink, increasing the size of the gas bubbles in the standpipe. As the gas bubbles cannot escape from the standpipe, and if they grow beyond a critical size, they can constrict and even block the ink flow thereby causing the printing quality to severely deteriorate.
Another undesired effect is the instability of the drop characteristic due to the gas present in the firing chamber. Some micro-bubbles formed in the previous boiling phases may remain in the chamber on the surface of the resistors. When the latter are fired, the micro-bubbles form nucleation points and, therefore, the next boiling phase starts at a lower and variable super-heating temperature due to the random distribution of the micro-bubbles. When the micro-bubbles are present, bubbles of vaporized ink having smaller and unsteady size are generated during printing. This effect causes an intermittent and random decrease of the drop mass and velocity of usually about 20 percent.
A standard print head for the consumer market, i.e. for home and office applications, is usually a disposable one. Basically, as illustrated in FIGS. 1 and 2, a print head cartridge 1 comprises a cartridge body or housing 2, usually made of plastics, that houses a suitable backpressure generating element 3, the latter being made of a porous material like foam or fiber, or a combination of them. The backpressure element 3 almost completely fills out the ink reservoir inside the housing 2 and the ink occupies the pores of the material, flowing through them towards a print head chip to reach the ejection sites.
A filter 4, usually made of metal, is fitted into the cartridge at the lower side of the backpressure element 3 and prevents possible debris or particles, produced during the manufacturing, from reaching the microfluidic circuit of the print head.
Beyond the filter 4, a standpipe 5 forms the flow path through which the ink travels, before reaching the feeding slots at the backside of the print head chip. A lid 6, which forms the top of the housing 2, acts as a cover for the cartridge 1.
The ink contained in the cartridge 1 is sufficient to allow regular printing over a limited, but for consumer market sufficiently long, period of time. The ink can be outgassed before being filled into the housing 2 of the cartridge 1. Frequently, the ink is not even outgassed. In any case, the total amount of dissolved gas in the ink, either already present in not-outgassed ink or captured from internal surfaces of the cartridge 1, e.g. the backpressure element 3, where gas can be adsorbed, normally does not have a significant effect on the printing performance. In fact, the volume of the accumulated gas that can be released from the liquid ink in form of bubbles is small with respect to the volume of the standpipe 5 through which the ink travels, moving towards the print head, which is attached at a lower surface 7 of the housing 2.
Therefore, the ink in the cartridge 1 can be completely consumed without the print head undergoing any serious criticality due to gas bubbles. Even in case of a refilled cartridge, the device lifetime typically allows just a few refills of the ink and the total volume of the gas bubbles remains relatively low. As a conclusion, the problem of the gas bubble formation in a print head device can be kept under control in a disposable cartridge or even a refill cartridge.
On the other hand, gas accumulation tends to cause severe issues when the same cartridge is refilled continuously from outside with an adducting pipe that sinks the ink from an external tank, such as a bottle, even if the ink has been previously outgassed.
The large volume of ink that flows through a continuously refillable cartridge during the long-time printing operations leads to a prolonged contact between the liquid and the internal environment of the cartridge, which results in a higher risk of increasing the amount of gas captured in the ink and subsequently dissolved. Therefore, the formation of air bubbles due to the periodic depressions during printing is augmented. Hence, the bubbles can increasingly grow until they reach a critical size that blocks or obstructs the ink flow through the stand pipe 5, causing a failure of the print head.
In addition, the gas bubble generation by extraction of dissolved gas in the ink takes place much easier and causes a much more severe criticality when solvent based ink is used instead of water based ink. In fact, solvents tend to capture and release a larger amount of gas and the drawbacks during the printing can arise in a short time.
A backpressure in the hydraulic circuit containing the liquid ink is necessary to prevent the ink from dropping out of the housing, which otherwise would be caused from the hydrostatic pressure exerted by the ink column in the housing 2. This backpressure can be provided by a backpressure element, for example a porous medium whose capillarity acts as a retaining force on the ink. The porous medium could be foam or another porous material such as a textile, or a combination of different materials, being able to adequately fill the internal space in the housing 2, while accurately matching the filter 4 on the bottom of the housing 2. The details of the backpressure element 3 depend very much on the ink composition, and very often such a constraint largely reduces the range of usable materials, if the ink is solvent based.
The capillary forces in the porous material of the backpressure element 3 are interface phenomena and they take place at the boundary surface between a liquid and a gas. Therefore, the backpressure element 3 would not exert any retaining force, or backpressure, if it was completely sunk in the liquid or, in other words, if the liquid covered it completely. It is necessary that at least a small upper portion of the porous backpressure element 3 is not covered by the liquid in order that the capillary forces are established and the necessary backpressure can be generated in the cartridge.
As is illustrated in FIG. 3, the housing is only filled up to a maximum level which is located below the lid 6, i.e. the top of the housing 2, and below the upper end of the backpressure element 3. An actual ink level 8 in FIG. 3 takes its maximum value, i.e. equals the maximum level. The volume inside the housing 2 below the lid 6 and above the actual ink level 8 contains only gas or vapor. In this way, at the transition surface between the liquid ink and the gas, a suitable boundary interface is formed in the porous material, generating the desired backpressure.
FIG. 4 depicts a conventional print head cartridge for a continuous printing system, i.e. a continuously refillable ink-jet cartridge. In a continuous printing system, a large amount of ink is ejected from the print head 9 disposed on the bottom of the housing 2 during a longtime operation. An external pipe 10 conveys the ink into the housing 2 from an external tank (not depicted). The external pipe 10 is normally connected to an inlet port 11 placed on top of an upper cover 12 which upper cover 12 is in turn attachable to the lid 6 of the cartridge by means of a latching system.
The cover 12 has engaging features and sealing gaskets so that it can easily be removed from the lid 6. The cover 12 engages with a suitable ink feeding inlet 13 of the lid 6, wherein a gasket 14 ensures tightness of the connection between the latched cover 12 and the lid 6. An adapter 15 can be fitted to both the inlet port 11 on the cover 12 and an end 16 of the external pipe 10 to allow an easy and leak-free connection between the external pipe 10 and the cover 12 which guides the ink via the feeding inlet 13 through the lid 6 into the housing 2.
In addition, the cover 12 can also provide electric contacts 17, which can be used to establish a connection with ink level sensing elements 18, so that feedback with respect to the ink level in the housing 2 can be provided through an electrical connector 19 of the refilling device, in order to control and ensure the ink flow.
A venting port can be provided in the lid 6 to keep the volume above the backpressure element 3 and more particular above the ink in the backpressure element 3 at atmospheric pressure thereby facilitating drawing any liquid from the housing 2.
When the continuously refilled cartridge 1 has reached the end of its lifetime, it can be replaced with a new one, and the cover 12 can be engaged with the lid 6 of the new cartridge 1.
FIG. 5 shows the assembled cartridge 1 for a continuous refilling system, and FIG. 6 depicts the full configuration of the cartridge 1 and the cover 12 as well as the external pipe 10 and the means to inject ink into the housing 2 in its operating configuration.
In the prior art, as is depicted in FIG. 7, the ink conveyed from the external pipe 10 into the housing 2 through the inlet 13 drops down from the bottom side of the lid 6, directly onto the top side of the backpressure element 3. The backpressure element 3 has a lower portion 31 soaked in ink and an upper portion 32 which, in turn, is located in a gas or vapor environment. The boundary between these portions 31, 32 represents the actual ink level 8, indicated with the dash-dotted line in FIG. 7. The ink flows through the gaseous environment just below the lid 6 in the upper part of the housing 2 and travels spreading through the upper portion 32 of the backpressure element 3 which contains the same gas. Therefore, in the first part of the travel path into the housing 2, the ink interacts with the gas, either in the space above the backpressure element, or through the pores and at the surface of the upper portion 32 of the backpressure element 3. An interaction region 21 is approximately indicated by the dotted oval in FIG. 7.
As mentioned above, the captured gas subsequently dissolved in the ink can finally be extracted and released in the standpipe 5 beneath the filter 4. FIG. 8 illustrates a part of the housing 2, the filter 4, the stand pipe 5 and the print head 9. The lower portion 31 of the backpressure element 3 is soaked with ink and contacts an upper side of the filter 4. Beneath the filter 4, there is the standpipe 5 which is in fluid communication with the underlying print head 9. When a portion of the dissolved gas is extracted by the depression caused by the print head, small gas bubbles can grow into the ink. These gas bubbles can hardly follow the regular ink flow towards the ejection sites of the print head 9 because their density is much lower than the density of the ink. Hydrostatic forces tend to push them upwards so that they remain trapped in the standpipe 5 below the filter 4.
During the longtime ink flow, a large amount of gas can accumulate and be captured and released subsequently in form of a big bubble 22 trapped by the filter 4. Small bubbles can merge or can increase their own size, causing the forming of the bigger gas bubble 22 that grows continuously until some printing failure occurs due to the bubble obstructing the ink flow path in the standpipe 5.
This problem is conventionally addressed by a special cartridge design providing a secondary channel where an extraction and elimination process is performed using additional valves and pumping devices. However, this solution significantly increases complexity and cost of the printing system. Further, semi-permeable filters must be used according to this solution to avoid the extraction of the ink with the gas bubbles and the pumping parameters must be accurately set within a suitable operating range to exploit effectively such a filtering action.